The Atomic Force Microscope is a well-known device in which the topography of a sample is sensed by a tip mounted on the end of a microfabricated cantilever. As the sample is scanned, the interaction of atomic forces between the tip and the sample surface causes pivotal deflection of the cantilever. The sample topography is determined by detecting this deflection.
The AFM technology has also been applied to the field of data storage with a view to providing a new generation of high-density, high data-rate data storage devices for mass-memory applications. AFM-based data storage is described in detail in IEEE Transactions on Nanotechnology, Volume 1, number 1, pages 39 to 55, Vettinger et al., and in IBM Journal of Research & Development, Volume 44, No. 3, May 2000, pp323-340, “The ‘Millipede’—More Than One Thousand Tips for Future AFM Data Storage”, Vettiger et al., and the references cited therein.
The cantilever-mounted tip, referred to as the read/write tip, is used for reading and writing of data on the surface of a data storage medium. In operation, the read/write tip is biased against the surface of the data storage medium. The storage medium generally comprises a polymeric material.
In the write mode, the read/write tip is heated which results in heat transfer to the polymer surface layer causing local softening of the polymer. This allows the tip to penetrate the surface layer to form a pit, or bit indentation; such a pit represents a bit of value “1”, a bit of value “0” being represented by the absence of a pit. This technique is referred to as thermomechanical writing. In an alternative system disclosed in U.S. Pat. No. 5,446,720 in the name of Canon Kabushiki Kaisha the value “0” is represented by pit representing a tracking bit and the value “1” by a pit representing an information bit, the depth of the tracking pit being significantly less than that of an information pit.
The storage medium can be moved relative to the read/write component tip allowing the tip to write data over an area of the surface, or “storage field”, corresponding to the field of movement. Each indentation is created by heating the cantilever tip and with the application of force pressing this tip into the polymer film. The tip is heated by passing a current through a resistive heater integrated in the cantilever, directly behind the tip. Some of the heat generated in the resistor is conducted through the tip and into the polymer film, locally heating a small volume of the polymer. If sufficient heat is transferred to raise the temperature of the polymer above a certain temperature (which is dependent on the chosen polymer), the polymer softens and the tip sinks in, creating an indentation or bit.
In the read mode, the storage field is scanned by the tip, the position of the tip and the cantilever on which it is mounted differs according to the presence or absence of a pit. The reading operation uses thermomechanical sensing based on the principle that the thermal conductance between the cantilever, and components attached thereto, and the storage substrate, changes according to the distance between them; the distance is reduced as the tip moves into a bit indentation. Further discussion of the reading operation can be found in the above identified IBM Journal of Research & Development article.
The early storage medium consisted of a bulk polycarbonate layer. The IBM Journal of Research & Development and IEEE Transaction on Nanotechnology articles disclose an improved storage medium comprising a silicon substrate having a very thin layer of polymer thereon. The preferred storage medium comprises a silicon substrate having a thin layer of polymethylmethacrylate (PMMA) as the read/write layer. The advantage of having a silicon substrate is that the hard silicon substrate limits the penetration of the tip and also, because silicon is a better conductor of heat than polymers such as PMMA, there is improved transport of heat away from the pits during the reading and writing processes. The PMMA layer is suitably about 40 nm thick thus giving a depth of pit of up to 40 nm. Problems of tip wear are believed to be caused by the tip penetrating the polymer layer and making contact with the hard silicon substrate, and in a further improved storage medium, a layer of crosslinked photoresist, in this example SU8 resin from MicroChem Corporation, Newton, Mass., USA, was introduced between the PMMA and the silicon substrate. The layer of crosslinked resin, typically about 70 nm thickness, acts as a softer penetration stop thereby reducing tip wear.
A data storage device will include the data storage medium described herein. In a multi-cantilever/tip device such as described in the Vettinger paper, above, multiple simultaneous operations can be carried on in a common polymer substrate by individually addressing each bit location. By virtue of the nanometer length-scale of each operation, this array of multiple bit locations in sum occupies a minimum amount of space constituting an ultrahigh density ‘reactor’. Data are stored by making nanoscopic indentations in a thin polymer film using a highly parallel array of cantilevers. As described above, at each position, an indentation or pit represents a 1 and no indentation or pit represents a 0, therefore data can be stored in a traditional binary sense via the presence or absence of nanoscopic indentations in the thin polymer film which serves as the storage medium.
The efficiency of writing and reading the indentations (bits of information) is therefore critically dependent on the nature of the polymeric thin film. Desirable attributes of the polymeric thin film are ‘softness’ and deformability during the writing phase, toughness and resistance to wear during the reading phase, and long term stability over the lifetime of the stored data. A hard polymer with a high melting point will be difficult to soften sufficiently for the tip to sink in and form the pit during the writing process. Conversely a hard polymer will be preferred during the reading process; the tip is required to travel across the polymer surface and the surface must be sufficiently hard and smooth to minimise the wear on the tip and damage to the surface. Finding a material with these properties is problematic since one feature normally precludes the other.
Linear polymers such as PMMA have been found to have suitable writing temperatures and the force required on the tip to form the pit is acceptably low for the required tip performance and power consumption; however, the wear rate on reading has been found to be unacceptably high because of the softness of the surface. Crosslinking of these polymers leads to a toughening of the surface and an improved tip wear rate during reading but requires a consequent increase in writing temperate and force leading to increased tip wear during the writing phase.
The present invention seeks to overcome these problems by using a class of polymers which under controlled conditions have the characteristics of linear polymers and are thus suitable for the writing phase but have the characteristics of crosslinked polymers during the conditions of the reading phase.